A known method of enhanced oil recovery for low gravity crude oil is the In-Situ Combustion method. This method requires a minimum of two oil wells, one used as an injection well and the other used as a production well.
Oxygen, oxygen-enriched air or merely air is injected through the injection well to the low gravity crude oil bearing formation. Subsequent to the injection of an oxygen supply, the low gravity crude oil is ignited downhole by methods known in the art.
Ignition of the air/crude oil mixture can also be accomplished by injecting heated air or by introducing a chemical into the oil-bearing reservoir rock.
This method produces heat energy by burning some of the oil within the reservoir rock itself. The amount of oil burned and the amount of heat created during in-situ combustion can be controlled to some extent by varying the quantity of air injected into the reservoir. Although the physics and chemistry of in-situ combustion are extremely complex, the basic principles are logical. Basically, the combustion heat vaporizes the lighter fractions of the crude oil and drives them ahead of a slowly moving combustion front created as some of the heavier, unvaporized hydrocarbons are burned. Simultaneously, the heat vaporizes the water in the combustion zone. The resulting combination of gas, steam and hot water aided by the thinning of the oil due to heat and the distillation of light fractions driven off from oil in the heated region moves the oil from injection to production wells.
The attractiveness of in-situ combustion lies in the fact that it requires the injection of only compressed air. Although the in-situ combustion method is applicable to a wide variety of reservoirs, its limitation lies in medium and high gravity crude oil reservoirs.
Medium and high gravity crude oil reservoirs have a very low boiling point along with low viscosity. When an in-situ combustion process is started, distillation of light fractions driven off from the oil in the heated region moves rapidly from the injection well to the production well. Since medium and high gravity crude oil is composed largely of light fractions having a very low boiling point, the vapors move from the combustion zone so rapidly that very little coke, which sustains combustion, is left behind. As a result, the in-situ combustion method for enhanced oil recovery cannot be used in reservoirs that are comprised mainly of medium or high gravity crudes since combustion cannot be sustained and will extinguish itself after a very short period.